Conventionally, as a suspension spring and an engine valve spring in which high strength and high fatigue strength are required, a titanium alloy which is generally classified as a β type, has superior cold workability and has high strength relatively easily by heat treatment, is primarily used, among Ti alloys used in parts for vehicles. The β type Ti alloy is an alloy having a composition classified as a Ti alloy that is age-hardenable after a metastable β phase at room temperature. However, since the β type Ti alloy is ordinarily an alloy in which the β phase, being stable at high temperatures, is treated so as to be metastable at room temperature by solution treatment, it is necessary to contain large amounts of β stabilizing elements such as V, Mo, and Cr, which are expensive. Therefore, Ti alloy parts having comparable strength and being made of inexpensive material has been greatly desired.
In addition, strength of a β type Ti alloy is improved by a heat treatment such as an α phase precipitation aging treatment; however, fatigue strength is important in mechanical parts in practical use. However, breaking of a β type Ti alloy would occur from cracking in an α phase particle precipitated or interface of an α phase and a β phase, and occurrence of the cracking in both cases is considered to be caused by differences in elastic strain between an α phase and a β phase. Therefore, in a structure that is strengthened by precipitation of an α phase from a β matrix phase such as a β type Ti alloy by aging treatment, there have been limitations in improving fatigue strength, even if static strength is superior. In view of such circumstances, application of a near α type or an α+β type Ti alloy, in which content of expensive β phase stabilizing elements is low and content of β phase which is deformed easily and has low strength is low, to vehicle parts, has been anticipated from the viewpoints of fatigue strength and cost.
On the other hand, as disclosed in the Japanese Patent No. 3789852, since Ti-6Al-4V (mass %) alloy, which is typically classified as an α+β type, has good balance in mechanical properties such as strength, ductibility and toughness, the penetration is large, accounting for about 70% in production amount of all Ti alloys. Therefore, Ti-6Al-4V alloy has advantages such as low cost and low variation in component and material strength.
Properties and strengths of such Ti-6Al-4V alloys are mainly affected by formation of structures, that is, whether the structure is made of equiaxial crystal structures, acicular crystals or mixtures thereof (bimodal structures) regarding formation of an α phase. Generally, the equiaxial crystal structure is formed by processing in a temperature range not more than β transus temperature−50° C. for example, and is superior in strength, elongation, generation resistibility of fatigue cracking and plastic workability. The acicular crystal structure is formed by processing in a temperature range not less than β transus temperature+50° C. for example, and is superior in creep resistance, breaking toughness and resistance to propagation of cracks. Furthermore, the mixture (bimodal) structure is formed by solution processing at a temperature just below β transus temperature and subsequent aging treatment at about 550° C., for example, and has both advantages of the equiaxial crystal structure and the acicular crystal structure.
However, it is difficult for the above-mentioned Ti-6Al-4V alloy to have properties superior to static strength of the above-mentioned β type Ti alloy, and in many cases, kinetic property and functional property thereof are controlled by controlling micron size structures and structure formation. However, in recent years, there have been attempts to control microstructures of metallic materials at the nanoscale by using a severe working method such as ECAP (Equal Channel Angular Pressing) method disclosed in “Materials, Vol. 37, No. 9 (1998), pp. 767-774 by Hotta et al.”, or ARB (Accumulative Roll-Bonding) method disclosed in Japanese Patent No. 2961263, and as a result, it has been found that metal having nanostructures can yield superior mechanical properties that conventional metallic materials cannot attain.
However, the ECAP method is a method in which a metallic bulk to be processed is repeatedly injected into and passed through a tunnel-like extrusion pathway having one bended part between entrance and exit, so as to give the metallic bulk much shear strain. In such a shear deformation processing method, since there is a limitation in length of the material that is supplied and processed, it is difficult to lengthen the material and to enlarge the apparatus, in principle.
Furthermore, the ARB method has an advantage in that a plate material can be processed in more than process limitation by repeating rolling of stacked rolled plate materials; however, the method can be applied only to a plate material, and cannot be applied practically to mechanical parts having complicated shapes.